Multifrequency signaling systems typically employ coincident tone bursts coded in terms of frequency for the transmission of signaling information. Such multifrequency signals may be generated by a pushbutton TOUCH-TONE telephone subset in which pushing a numbered button produces two simultaneous tone bursts having frequencies in Hz as follows:
______________________________________ Button No. Low Freq. High Freq. ______________________________________ 1 697 1209 2 697 1336 3 697 1477 4 770 1209 5 770 1336 6 770 1477 7 852 1209 8 852 1336 9 852 1477 0 941 1336 ______________________________________
Pairs of tone bursts are transmitted over a telephone line as a person presses buttons indicating the number of a remote telephone being "dialed." A TOUCH-TONE receiver at a switching point translates the pairs of tone bursts back to signals representing numbers 0 through 9 for operation of the switching equipment.
In prior art multifrequency receivers, a high band filter is used to pass the high frequency band tones to one path in which an additional path filter is provided for passing each one of the specific high band tones. Similarly, a low band filter is used to pass the low frequency band tones along another path in which an additional filter is provided for passing each one of the specific low band tones. The additional filters are used to detect which tones are present.
The additional filters must each be designed to pass solely the intended frequency tone, and to block other frequency tones. The filters are expensive and less than entirely satisfactory because they must be carefully adjusted at the factory with the hope that they will remain adjusted in use and not have changing characteristics due to environmental conditions such as changing temperatures and power supply voltages.